Hitherto, silver paste has been mainly used in order to bond a semiconductor element and a semiconductor-element-mounting supporting member to each other. However, as the size of semiconductor elements has been made smaller and the performance thereof has been made higher in recent years, supporting members used therein have also been required to be made smaller and minuter. Against such requirements, the following have been caused according to silver paste: at the time of wire bonding, an inconvenience is caused by a matter that the paste is pushed or forced out or by an inclination of the semiconductor element; the film thickness of the adhesive sheet is not easily controlled; voids are generated in the adhesive sheet; and others are caused. Thus, it has been becoming difficult that silver paste copes with the requirements. For this reason, in order to cope with the requirements, the use of an adhesive in a sheet form has been spreading in recent years.
This adhesive sheet is used in an individual-piece-adhering manner or in a wafer-rear-surface adhesion manner. When the former, i.e., the individual-piece-adhering manner adhesive sheet is used to produce a semiconductor device, the adhesive sheet, which is in a reel form, is cut or punched into individual pieces. Thereafter, any one of the pieces is bonded onto a supporting member. Onto the resultant adhesive-sheet-attached supporting member is then bonded one out of individual semiconductor elements divided in a dicing step, so as to produce a semiconductor-element-attached supporting member. Thereafter, the member is optionally caused to undergo a wire bonding step, a sealing step, and some other step, thereby yielding the semiconductor device. However, in order to use the individual-piece-adhering manner adhesive sheet, it is indispensable to cut the adhesive sheet into the individual pieces, and a special fabricating apparatus for bonding any one of the pieces onto the supporting member is required. Thus, there remains a problem that costs for the production are higher than according the method using silver paste.
On the other hand, when the latter, i.e. the wafer-rear-surface adhesion manner adhesive sheet is used to produce a semiconductor device, the semiconductor device is yielded through the following process: first, the adhesive sheet is adhered onto the rear surface of a semiconductor wafer and further a dicing tape is adhered onto the other surface of the adhesive sheet; thereafter, the wafer is diced into individual semiconductor elements; any one of the adhesive-sheet-attached semiconductor elements is picked up, and the element is bonded onto a supporting member; and the workpiece is caused to undergo heating, curing and wire bonding steps, and some other step. This wafer-rear-surface adhesion manner adhesive sheet does not require any device for dividing the adhesive sheet into individual pieces in order to bond the adhesive-sheet-attached semiconductor element onto the supporting member. Thus, a conventional fabricating apparatus for silver paste may be used as it is, or the apparatus may be used in the state that the apparatus is partially improved, for example, a heat plate is added thereto. For this reason, attention has been paid to the method, as a method for controlling costs for the production into a relatively low level, out of fabricating methods using an adhesive sheet.
However, in the method using a wafer-rear-surface adhesion manner adhesive sheet, it is necessary that the adhesive sheet is also cut in the step of dicing the wafer. Examples of the method for cutting this adhesive sheet include a contact-manner cutting method of attaining the cutting using a diamond blade; a method of irradiating the wafer with a laser to form a reformed region selectively inside the wafer, and subsequently expanding the wafer, whereby the adhesive sheet is cut at the same time when the wafer is cut along the reformed region; and a method of cutting the wafer, adhering the adhesive sheet onto the cut wafer, and then expanding the wafer to cut the adhesive sheet along the wafer-cut lines (see, for example, Japanese Patent Application Laid-Open No. 2006-093213). In any one of the methods, however, it is said to be effective to add an inorganic filler to the adhesive sheet so as to adjust the sheet into an appropriate hardness, thereby improving the cuttability thereof since it is difficult to cut the wafer, which is an inorganic material and is hard, and the adhesive sheet, which is an organic material and is soft, according to the same step.
One of the most important properties for mount boards, on each of which various electronic components, a typical example of which is a semiconductor element, are mounted, is reliability. In particular, connection reliability against thermal fatigue is a very important item since the reliability is related directly to the reliability of an instrument using a mount board. A cause for lowering this connection reliability is thermal stress generated from a matter that various materials different from each other in thermal expansion coefficient are used. Since the thermal expansion coefficient of semiconductor elements is as small as about 4 ppm/° C. while the thermal expansion coefficient of wiring boards, on each of which electronic components are to be mounted, is as large as about 15 ppm/° C. or more, thermal strain is generated against thermal impact so that thermal stress is generated by the thermal strain. This thermal stress causes a decline in the connection reliability. For this reason, it is a theme for adhesive sheets to relieve this thermal stress. Such a wiring board generally has irregularities based on wiring. An adhesive sheet needs to be embedded into the irregularities in this wiring board when bonded thereto. For any adhesive sheet for a semiconductor, from the viewpoint of the thermal-stress-relieving performance thereof and the embeddability into irregularities in a wiring board, and other viewpoints, it is desired that the elastic modulus is somewhat low after the curing of the sheet. However, it is known that when an inorganic filler is added thereto in order to improve the cuttability of the adhesive sheet, the elastic modulus of the adhesive sheet generally becomes high. Thus, it becomes a theme to make the reliability of a semiconductor package and the cuttability of its adhesive sheet compatible with each other.
An object of the invention is to provide an adhesive sheet for a semiconductor and a dicing tape integrated adhesive sheet for a semiconductor that are each good in cuttability when the sheets are each cut into individual pieces by expansion and are each good in embeddability into irregularities in a wiring board when the sheets are each bonded thereto, so as to exhibit an excellent reliability.